The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles suspended in a dielectric solvent. An EPD typically comprises a pair of opposed, spaced-apart plate-like electrodes. At least one of the electrodes, typically on the viewing side, is transparent. An electrophoretic fluid composed of a dielectric solvent and charged pigment particles dispersed therein is enclosed between the two electrode plates. When a voltage difference is imposed between the two electrode plates, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate, determined by selectively charging the plates, can be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color.
Known techniques for an electrophoretic fluid either disperse one type of charged pigment particles in a solvent of a contrast color or disperse two types of charged pigment particles of contrast colors in a clear solvent. In the former case where white charged particles are dispersed in a dark colored solvent, the whiteness displayed by the display device is limited by absorption of light in the interstitial locations between the white charged particles and by the amount of white particles that can go into the fluid before they become too low in mobility, due to field shielding and high viscosity of the fluid. In the latter case where both black and white particles are dispersed in a clear solvent, the whiteness is also limited due to the number of white particles and how well they can be separated from the black particles.